The present invention relates to gas or combustion turbine apparatus, gas turbine electric power plants and control systems and operating methods therefor.
Industrial gas turbines may have varied cycle, structural and aerodynamic designs for a wide variety of uses. For example, gas turbines may employ the simple, regenerative, steam injection or combined cycle in driving an electric generator to produce electric power. Further, in these varied uses, the gas turbine may have one or more shafts and many other rotor, casing, support and combustion system structural features which can vary relatively widely among differently designed units. They may be aviation jet engines adapted for industrial service as described for example in an ASME paper entitled "The Pratt and Whitney Aircraft Jet Powered 121 MW Electrical Peaking Unit" presented at the New York Meeting in November-December, 1964.
Other gas turbine uses include drive applications for pipeline or process industry compressors and surface transportation units. An additional application of gas turbines is that which involves recovery of turbine exhaust heat energy in other apparatus such as electric power or industrial boilers or other heat transfer apparatus. More generally, the gas turbine air flow path may form a part of an overall process system in which the gas turbine is used as an energy source in the flow path.
Gas turbine electric power plants are usable in base load, mid-range load and peak load power system applications. Combined cycle plants are normally usable for the base or mid-range applications while the power plant which employs a gas turbine only as a generator drive typically is highly useful for peak load generation because of its relatively low investment cost. Although the heat rate for gas turbines is relatively high in relation to steam turbines, the investment savings for peak load application typically offsets the higher fuel cost factor. Another economic advantage for gas turbines is that power generation capacity can be added in relatively small blocks such as 25 MW or 50 MW as needed for expected system growth thereby avoiding excessive capital expenditure and excessive system reserve requirements. Further background on peaking generation can be obtained in articles such as "Peaking Generation", a Special Report of Electric Light and Power dated November, 1966.
Startup availability and low forced outage rates are particularly important for peak load power plant applications of gas turbines. Thus, reliable gas turbine startup and standby operations are particularly important for power system security and reliability.
In the operation of gas turbine apparatus and electric power plants, various kinds of controls have been employed. Relay-pneumatic type systems form a large part of the prior art, but have heretofore not provided the flexibility desired, particularly in terms of decision making. Furthermore, such prior art systems have been characterized by being specially designed for a given turbine plant, and accordingly are not adaptable to provide different optional features for the user. More recently, electronic controls of the analog type have been employed as perhaps represented by U.S. Pat. No. 3,520,133 entitled Gas Turbine Control System and issued on July 14, 1970 to A. Loft or by the control referred to in an article entitled "Speedtronic Control, Protection and Sequential System" and designated as Ger-2461 in the General Electric Gas Turbine Reference Library. See also U.S. Pat. No. 3,662,545, which discloses a particular type of analog acceleration control circuit for a gas turbine and U.S. Pat. No. 3,340,883, relating to an analog acceleration, speed and load control system for a gas turbine. A wide variety of controls have been employed for aviation jet engines including electronic and computer controls as described for example in a March, 1968 ASME Paper presented by J. E. Bayati and R. M. Frazzini and entitled "Digatec (Digital Gas Turbine Engine Control)", an April 1967 paper in the Journal of the Royal Aeronautical Society authored by E. S. Eccles and entitled "The Use of a Digital Computer for On-Line Control of a Jet Engine", or a July, 1965 paper entitled "The Electronic Control of Gas Turbine Engines" by A. Sadler, S. Tweedy and P. J. Colburn in the July, 1967 Journal of the Royal Aeronautical Society. However, the operational and control environment for jet engine operation differs considerably from that for industrial gas turbines.
The aforereferenced U.S. application Ser. No. 319,114, assigned to the present assignee, presents an improved system and method for operating a gas turbine with a digital computer control system. In this system, one or more turbine-generator plants are operated by a hybrid digital computer control system, wherein logic macro instructions are employed in programming the computer for logic operations of the control system.
In referencing prior art publications or patents as background herein, no representation is made that the cited subject matter in the best prior art.
While industrial gas turbine apparatus and gas turbine power plants have attained a great sophistication, there remain certain operational limitations in flexibility, response speed, accuracy and reliability. Further limits have been in the depth of operational control and in the efficiency or economy with which single or multiple units are placed under operational control and management. Limits have existed on the economics of industrial gas turbine application and in particular on how close industrial gas turbines can operate to the turbine design limits over various speed and/or load ranges.
In gas turbine power plants, operational shortcomings have existed with respect to plant availability and load control operations. Compressor surge control response has been limited, particularly during startup. Temperature limit control has been less protective and less responsive than otherwise desirable.
Generally, overall control loop arrangements and control system embodiments of such arrangements for industrial gas turbines have been less effective in operations control and systems protection than is desirable. Performance shortcomings have also persisted in the interfacing of control loop arrangements with sequencing controls.
With respect to industrial gas turbine startup, turbine operating life has been unnecessarily limited by conventional startup schemes. Sequencing systems have typically interacted with startup controls less effectively than desirable from the standpoint of turbine and power plant availability. More generally, sequencing systems have provided for systematic and protective advance of the industrial gas turbine operations through startup, run and shutdown, but in doing so have been less efficient and effective from a protection and performance standpoint than is desirable.
Restrictions have been placed on operations and apparatus management particularly in gas turbine power plants in the areas of maintenance and plant information acquisition. Further management limits have existed with respect to plant interfacing with other power system points, operator panel functionality, and the ability to determine plant operations through control system calibration and parameter changes.
The computerized gas turbine control as disclosed in U.S. application Ser. No. 319,114 has been highly successful in providing control capability and flexibility of control options that had not previously been incorporated into an all hardware type system. However, while the computerized, or software control system provides substantial advantages due to its logic performing capability, historical data storage and diagnostic programs, it also has a number of shortcomings. The interface between the turbine and its associated analog signals and the computer controller presents areas for future development and improvement. The analog input system is a complex multiplexing arrangement requiring sharing of the scan time by the variables which must be scanned or read "independently". In the system disclosed, there is a scanning rate of 30 per second, meaning that 30 input variables per second can be read, imposing a limitation on the ability of the system to respond rapidly to a given input variable when program running time is also added to the delay. In addition, the computer system itself incorporates elaborate techniques of self-diagnosis of failure, which can result in turbine shutdowns when the computer has determined that something has failed within the central processor, input-output, or peripheral hardware. It is most difficult for the computer to determine whether the failure is of a sufficiently critical nature to require shutdown. In fact, it has been found that failures in the analog input-output system may not be readily differentiated, leaving the computer no choice but to shut down the entire turbine system for a failure which may not justify loss of load availability. Since all monitoring and protection paths are channeled through a central processor, a self-determination of failure in the central processor, analog input multiplexing or output system by the computer controller necessitates blocking off all channels, such that complete system shutdown is required. Furthermore, even during normal operation, the computerized system provides low visibility with respect to the health of the control system. The essential intermixing of the control paths through the central processor makes it difficult for the operator to obtain information as to the mode of control at any moment, or to obtain quantitative information as to the relative magnitudes of the different control signals. In short, the increased flexibility of the software system is achieved at the expense of operator visibility such as permits optimum maintenance procedures. Accordingly, there is a great need in the art for a turbine system having a control with the logic capability of a digital system, but retaining the advantages which are inherent in simpler designs.
The gas turbine control system as disclosed herein incorporates novel features which are specifically designed to meet the above general requirements, and which go further in providing operating capability not heretofore available in any turbine control system. The control system of this invention includes a plurality of continuously closed control loops, each of which continuously generates a control signal adapted to control the turbine fuel system, and thereby control available fuel to the turbine, thereby controlling turbine operation itself. Each of the control loops contains logic capability, is adaptable to be constructed in different hardware forms, and provides continuous visual indication for the operator and continuous monitoring for alarm or turbine shutdown. In this manner, should failure, or even a lesser malfunction, occur in any of the control paths, a backup control signal is available to take over turbine control, without the failure causing loss of turbine availability. Furthermore, means are provided by which the operator can immediately determine the source and, in many instances, the nature of the malfunction, so that corrective maintenance can be quickly and efficiently undertaken.
Another specific improvement is the provision of adapting the turbine control for changes in ambient temperature, such as occur between summer and winter operation. In prior art systems, which are dependent solely on monitoring of internal turbine conditions, unwanted operating limitations are imposed by changes in ambient temperature. Such limitations have been reduced substantially by the novel adaptive control means disclosed herein.
Another area of great importance in gas turbine control is that of immediately meeting load demand upon generator breaker closing. Past controls have generally provided for a continuous buildup of load, starting from zero load at generator breaker closing and proceeding roughly linearly to a desired load level. However, there are a number of applications where it is required, or at least highly desirable, to provide an essentially instantaneous pickup of load. Accordingly, this invention provides novel means for controlling the turbine fuel flow so as to provide capability for such immediate load pickup.
A critical portion of the operation of any turbine involves the starting sequence, at which time the turbine undergoes severe temperature changes, with possible resulting damage due to thermal stress. The turbine control system of this invention accordingly incorporates novel features to limit turbine speed change as a function of monitored turbine termperature, and to schedule the fuel supplied to the turbine combustor element so as to minimize risk of thermal damage during the starting operation. One of the novel techniques employed in this respect is the specific means of scheduling bypass fuel flow in the turbine fuel system during startup, so as to control the fuel pressure at the combustor nozzles. A bypass temperature limiter valve suitable in this operation, with which desired combustor nozzle fuel pressure is obtained during ignition, is disclosed in co-pending U.S. patent application Ser. No. 261,192, assigned to the same assignee. The technique disclosed herein involves novel means of utilizing apparatus such as is disclosed in Ser. No. 261,192.
One of the greatest needs in any turbine control system is that of providing operator flexibility, and in particular providing the operator with the capability of efficiently changing load as desired. Most prior art turbine control systems are quite limited in the degree of flexibility available to the operator, e.g., only discrete operating load levels are available, or the available means of changing the load level to a desired level is cumbersome and/or cannot be achieved at a desired rate. Accordingly, the control system of this invention provides novel means having essentially unlimited flexibility for operator change of the load level, rate of change of such load level, the ability to hold load at any desired level, and the ability to return to any predetermined load level. This capability is constantly backed up by temperature control capability, such that no matter what the operator attempts to do, operational limits are automatically imposed as a function of sensed turbine temperatures.
A yet further need in a turbine control system is that of providing a reliable and workable monitoring system. As pointed out hereinabove, in computerized turbine control systems all monitoring and protection paths are channeled through a central processor, which frequently results in the computer requiring complete shutdown when, in fact, the turbine is being operated within safe limits. Also, even during normal operation, the computerized system frequently does not permit the degree of monitoring visibility which is highly advantageous for providing the operator with optimum ability to oversee the turbine operation. In order to overcome these difficulties and provide improved visibility and reliability, while maintaining maximum turbine availability, there is a need for a control system designed so as to provide continuous visual indication as to the current mode of turbine control, so that the operator can determine the health of the control system. This may be achieved by providing discrete modularized control paths which are in constant communication with the turbine and which generate independent control signals, and means for determining and displaying which of said independent signals is at any given moment in control of the turbine operation.
Turbine availability may also be greatly enhanced by providing means for automatically restarting the turbine after an automatic shutdown, and upon a determination that safe conditions exist for such restarting. In many cases, the condition which caused the turbine to be placed in shutdown is corrected, or corrects itself, shortly after shutdown is initiated. However, in conventional turbine control systems the turbine must be brought substantially completely to the shutdown state, and then restarted all over again. It is clear that this results in avoidable loss of turbine availability, and that there is a need to minimize the loss of availability by restarting as soon as turbine conditions permit. The gas turbine monitoring system as disclosed herein provides a shutdown subsystem having a novel arrangement for automatically restarting the turbine after correction of the malfunction which caused shutdown.
The monitoring system of this invention also incorporates a unique system and method for optimizing load availability while providing an alarm or other means to alert the operator to the existence of a control malfunction which must be corrected but which does not merit immediate turbine shutdown. This unique system avoids the inflexibility of currently used monitoring circuits which are designed either to be fail-safe (in which case load availability is sacrificed to ensure shutdown), or which are designed to fail in a designated direction, (thus always providing continuing availability, but at the cost of not shutting down in instances where shutdown might be required).